The present invention relates to continuous measurement-while-drilling surveying methods and apparatuses.
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached at a drill string end. A large proportion of the current drilling activity involves directional drilling, i.e., drilling deviated and horizontal boreholes to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth""s formations. Modem directional drilling systems generally employ a drill pipe having a drill bit at the bottom that is rotated by a drill motor or a xe2x80x9cmud motorxe2x80x9d. Pressurized drilling fluid or xe2x80x9cmudxe2x80x9d or xe2x80x9cdrilling mudxe2x80x9d is pumped into the drill pipe to rotate the drill motor and to provide lubrication to various members of the drill string including the drill bit. The drill bit and drill motor form part of what is known as the bottom hole assembly (xe2x80x9cBHAxe2x80x9d). As required the drill pipe is rotated by a prime mover, such as a motor, to facilitate directional drilling and to drill vertical boreholes.
Measurement-While-Drilling (MWD) surveying for directional and horizontal drilling processes is performed to provide the orientation and the position of the BHA [Conti, 1999]. Azimuth, the inclination and the tool face angles determine the orientation of the BHA, while latitude, longitude and altitude determine the position of the BHA. The altitude directly determines the true vertical depth of the BHA. State of the art MWD surveying techniques are based on magnetic surveying which incorporates three-axis magnetometers and three-axis accelerometers arranged in three-mutually orthogonal directions. The three-axis accelerometers monitor the Earth gravity field to provide the inclination and the tool face angles. This information is combined with the magnetometer measurements of the Earth magnetic field to provide the azimuth [FIG. 1, Russel et al., 1979].
The magnetic surveying system determines the BHA orientation at certain survey stations with the assumption that the error which modifies the Earth""s magnetic field vector at the surveying instrument is in the direction of the borehole [Russel et al., 1979]. This assumption is justified by installing these magnetometers inside a non-magnetic housing. Such housing system necessitates the use of non-magnetic drill collars around the surveying equipment at a cost approaching $30,000 per single installation [Rehm et al., 1989].
Although simple, the magnetic surveying system suffers from several inadequacies especially within the drilling environment [Thorogood, 1990; Thorogood et al., 1986]. The presence of downhole ore deposits deviate the measurements of the Earth magnetic field even with the non-magnetic drill collars surrounding the surveying instruments. In addition, magnetic surveying tools located in non-magnetic drill collars are subject to the influence of the other steel components of the drill string. It has been shown that drill string induced surveying error increases with inclination and as the borehole direction approaches the east-west azimuth. Drill string magnetic interference is particularly noticeable when inclination exceeds 30xc2x0. Although it has been reported that the effect of the drill string magnetic interference could be reduced (mitigated but never entirely eliminated) by running long lengths of non-magnetic materials above and below the survey instruments [Grindord et al., 1983], this solution could affect the cost benefits of the horizontal drilling technology. The effect of drill string magnetic interference as well as the presence of downhole ore deposits can neither be quantified nor compensated. In addition to these two effects, geomagnetic influences play an important role in the accuracy of the magnetic surveying system [Parkinson, 1983]. Geomagnetic influences are defined by the variation of the dip angle, the declination and the total magnetic field strength with respect to time. The dip angle is the angle between the direction of the Earth""s magnetic field and the horizontal plane. The declination is the angle between the magnetic north and the true north. It was recorded that during any given day at a random location the standard deviations of the dip angle, the declination and the magnetic field strength were 0.3xc2x0, 0.9xc2x0 and 0.3 xcexc-Tesla respectively [Parkinson, 1983]. The variation in the geomagnetic field is quite significant in relation to the performance capabilities of the magnetic surveying tools currently used. Therefore, geomagnetic effects must be taken into account when considering absolute survey accuracy and impose definite limitations on the accuracy levels that can ultimately be achieved.
Several investigations have been reported to improve the magnetic surveying accuracy. Shiells and Kerridge (2000) introduced the interpolated in-field referencing method in which absolute local geomagnetic field data is determined from spot measurements of the Earth""s magnetic field. These measurements are taken at local site which is sufficiently close to the drilling site so that the measurement data is indicative of the Earth""s magnetic field at the drilling site but is sufficiently remote from the drilling site so that the measurement data is unaffected by magnetic interference from the drilling site. This technique compensates for the geomagnetic influences but the measurements taken by the magnetometers still suffer from the drill string magnetic interference. In addition, measurements of the geomagnetic field in a local site might not be applicable in an area that has many drilling rigs. Moreover, such technique adds more complexity that may give rise to additional cost.
Hartmann (1998) introduced a method to improve the accuracy of borehole surveying. This method is based on determining the uncertainty in the magnetic field measurements by comparing the measured magnetic field with a corresponding known value at specific location. However, this method does not take into account two phenomena. The first phenomenon is the increase of the drill string magnetic interference at high inclinations. The second is the presence of ore deposits that can be found randomly downhole. These two phenomena add additional errors that are not taken into consideration when referencing the theoretical value of the magnetic field to the measured value at the known location.
Dipersio (1995) introduced a new method for the compensation of geomagnetic influences. This method depends on matching both the calculated magnetic field strength and the calculated dip angle to their corresponding nominal values at a particular geomagnetic location. This results in determining an error-free value of the axial component of the magnetic field which is directly used to determine the azimuth. However, at high inclinations, the drill string magnetic interference generates additional errors in the cross-axial directions.
Several other techniques have been described for magnetic MWD surveying [Nicholson, 1995; Engebretson, 1992; Helm, 1991; van Dongen et al., 1987; Trowsdale, 1982]. However, it is clear that the major weaknesses of the present directional sensing instruments stem from the use of magnetometers for monitoring the azimuth and from the hostile environment in which these devices work. The problem encountered with the use of magnetometers is the presence of massive amount of steel around the drilling rig. The abundance of ferromagnetic material necessitates the separation of the magnetometers by non-magnetic drill collars. Aside from the cost of utilizing non-magnetic drill collars, their use introduces a second problem. Since the non-magnetic drill collars impose an additional weight on the drill bit, the surveying tools are separated from the bearing assembly and the drill bit by about 30 feet [Conti, 1989; Rehm at al., 1989]. Elimination of the non-magnetic drill collars could reduce the distance between the instrument package and the drill bit by approximately 50%. The third problem associated with the use of magnetometers is their lack of reliability when used underground due to the deviation of the Earth""s magnetic field from ore deposits.
It is known to use strapdown inertial navigation systems for borehole surveying using probes. Borehole surveying is performed after finishing the drilling process, or sections of the drilling process, for the purpose of quality assurance. U.S. Pat. No. 4,542,647 describes a borehole inertial guidance system which utilizes two ring laser gyroscopes having their sensitive axes normal to the borehole axis. In this case, azimuth is determined by a synthetic rotation signal provided by three-axis accelerometers. While this configuration may be suitable for borehole mapping, it fails to be useful for MWD surveying. There is considerable rotation about the tool-spin axis in a MWD process making the use of a synthetic signal to monitor this rotation and therefore azimuth useless.
In addition, ring laser gyros are unsuitable for MWD processes because they cannot withstand the harsh conditions encountered near the BHA during the drilling process. In particular, the vibration and shock forces associated with the progress of the drill bit impairs the effectiveness of ring laser gyros.
Therefore, there is a need in the art for a continuous measurement-while-drilling surveying method and apparatus which mitigates the difficulties of the prior art.
The present invention relates to the replacement of the MWD magnetic surveying system with the technology of inertial navigation. Inertial navigation systems (INS) determine the position, velocity and orientation of a moving body in three-dimensional space by integrating the measured components of the acceleration and the angular velocity of the body [Titteron et al., 1997]. The measurements of acceleration and angular velocity are provided by three-axis accelerometers and three-axis gyroscopes arranged in three mutually orthogonal directions.
Currently available 3-gyroscopes/3-accelerometers INS technology cannot provide high accuracy and small dimensions within the same setup and therefore at present, the direct installation of such system inside the BHA to replace the set of magnetometers is not feasible. Thus, the traditional high accuracy INS setup is not applicable downhole due to the limited space inside the BHA and the relatively large dimensions of the gyroscopes.
Therefore, herein disclosed is a method and apparatus for downhole strapdown INS utilizing only two novel gyroscopes. One gyroscope has its sensitive axis along the tool spin axis while the other has its sensitive axis normal to the tool spin axis and along the forward direction. This results in the removal of the costly non-magnetic drill collars because the gyroscopes are insensitive to magnetic fields. In one embodiment, the two gyroscopes are mounted inside the bearing assembly and are fiber optic gyroscopes (FOG). FOGs are suitable for the downhole drilling application because they do not contain moving parts, thus providing high reliability with no need for frequent calibration or maintenance [Kim, 1994; Merhav, 1993; Lefevre, 1993]. In addition, the FOGs exhibit low environmental sensitivity since they can withstand relatively high temperatures, shocks and vibrations [Noureldin et al., 1999; Noureldin et al., 2000]. Moreover, currently available FOGs are of small size, with a drift rate less than 1xc2x0/hr, long mean time between failure (60,000 hrs), no gravitational effects and excellent immunity to vibration and shock forces.
In one embodiment, the FOG that has its sensitive axis along the tool spin axis is prepared as torus to accommodate the flow of drilling fluid through the bearing assembly. The 3 accelerometersxe2x80x942 FOG system is capable of providing continuous MWD surveying during the whole drilling process, up until the horizontal section of the well, instead of MWD station-based surveying. In addition, continuous MWD surveying is possible through an initial radical section of the well with 3 accelerometers and a single FOG which has its sensitive axis aligned with the tool spin axis. The initial radical section of the well may be that portion with an inclination less than about 45xc2x0, preferably less than about 30xc2x0 and more preferably less than about 20xc2x0.
Therefore, in one aspect, the invention comprises a continuous measurement-while-drilling surveying apparatus for surveying the drilling progress of a bottom hole assembly (xe2x80x9cBHAxe2x80x9d) comprising:
(a) a housing adapted to be part of the BHA, said housing having a tool-spin axis;
(b) a first fiber-optic gyroscope mounted within the housing for generating a first angular rotation signal representing angular rotation of the BHA about the tool-spin axis;
(c) a second fiber-optic gyroscope mounted within the housing for generating a second angular rotation signal representing angular rotation of the BHA about an axis normal to the tool-spin axis;
(d) accelerometer means for generating three acceleration signals representing the components of acceleration of the BHA along three mutually orthogonal axes;
(e) first processing means responsive to the acceleration signals for determining the angle of the BHA away from the vertical and for generating a third angular rotation signal representing rotation of the BHA about an axis normal to the sensitive axes of the first and second gyroscopes;
(f) second processing means responsive to the first, second and third angular rotation signals and the acceleration signals for transforming signals representing movement of the BHA in a BHA coordinate system to a earth local-level coordinate system;
(g) third processing means operatively connected to the second processing means for determining the orientation of the BHA, determining the velocity changes of the BHA, updating the velocity components of the BHA and updating the position components of the BHA.
The first gyroscope may be toroidal. The first, second and third processing means may be a general purpose computer programmed with appropriate software, firmware, a microcontroller, a microprocessor or a plurality of microprocessors, a digital signal processor or other hardware or combination of hardware and software known to those skilled in the art.
In another aspect of the invention, the invention is a method of continuous MWD surveying of a wellbore which includes, on completion, a vertical section, a radical section and a horizontal section, using a BHA comprising a first gyroscope having its sensitive axis aligned with the tool spin axis, a second gyroscope having its sensitive axis on an axis normal to the tool spin axis and three accelerometers arranged in three mutually orthogonal directions, said method comprising the steps of:
(a) receiving the rotation signals from the first gyroscope and the second gyroscope and the acceleration signals from the three accelerometers;
(b) establishing the desired azimuth direction;
(c) determining the pitch angle from the accelerometer signals;
(d) determining the rotation rate about an axis normal to the sensitive axes of the two gyroscopes by determining the time rate of change of the pitch angle;
(e) determining the effect of the earth rotation and local-level (l-frame) change of orientation to the angular changes monitored along the three axes and removing these two effects from the rotation rate signals;
(f) transforming the rotation signals from a BHA coordinate frame to the earth local-level coordinate frame;
(g) determining the pitch, roll and azimuth of the BHA;
(h) determining the time rate of change of the velocity of the BHA expressed in the l-frame and update the velocity components of the BHA; and
(i) updating the position components of the BHA.
In one embodiment, this method is used for an initial portion of the radical section of the wellbore. The initial portion of the radical section of the well may be that portion with an inclination less than about 45xc2x0, preferably less than about 30xc2x0 and more preferably less than about 20xc2x0. After the initial portion, the method is altered by switching the axes orientation of the two gyroscopes. Once the wellbore reaches the horizontal section and rotary mode drilling is required, the surveying method may then switch to station-based surveying.